(1) Field of the Invention
The present invention relates to a process for producing biaxially drawn plastic bottles having excellent heat resistance capable of retaining their shapes during the periods in which they are filled with liquids of high temperatures or during the heat treatment for sterilizing the contents.
(2) Description of the Prior Art
A process has been known in which a secondary molded article obtained by biaxially drawing and blow-molding a preformed article is heat-treated (thermally set) and the heat-treated article is finally blow-molded in a mold in order to impart heat resistance to the biaxially drawn plastic bottles, so that they can withstand filling of liquids of high temperatures or the step of sterilizing the contents.
According to, for instance, Japanese Patent Publication No. 39416/1992, the barrel of an intermediate container obtained by blow-molding a preformed article is heated at 180.degree. to 220.degree. C. for 1 to 15 minutes to effect the heat crystallization and, in this case, the heating is accomplished in an atmosphere or by the hot air of 180.degree. to 220.degree. C.
According to Japanese Laid-Open Patent Publication No. 78728/1988, a step of thermal shrinking is provided between the step of final draw-molding and the step of primarily draw-molding the preform or the step of deforming the bottom, and in FIG. 9 thereof is shown a method of thermally shrinking the barrel of an intermediate container obtained by blow-molding a preformed article by using a plurality of linear heaters that are combined together maintaining a suitable gap.
Moreover, Japanese Laid-Open Patent Publication No. 122516/1988 teaches effecting the first stage of draw-molding based on a free blow-molding without using metal mold and effecting the intermediate heating by using an infrared ray heater in the step of effecting two stages of draw-molding and in the step of intermediate thermal shrinking.
According to the conventional method of effecting two stages of draw-molding and the intermediate thermal shrinking, however, the temperature for heat-treating the intermediate molded particle tends to become locally uneven causing the final product to exhibit insufficient heat resistance. Moreover, the heat treatment is carried out requiring an extended period of time. Therefore, the residence time becomes long in the whole apparatus, and the production efficiency is low.
That is, according to the prior art quoted first, the temperature of the intermediate container rises from the surface due to the conduction of heat and arrives at a predetermined temperature requiring a relatively long period of time. Therefore, the facility of a relatively large scale is required for heating the intermediate container, and the production efficiency is low.
It is widely accepted practice to heat the molded articles or the preformed articles of a plastic by using a plurality of linear infrared-ray heaters. In this case, though the individual infrared-ray heaters emit large amounts of radiant energy, the amount of radiant energy on the surface of the material being heated is of a level of smaller than 20% that on the surface of the heat-radiating members since the radiant heat is diffused from the linear heaters and, hence, the heating is accomplished requiring a relatively extended period of time. In the case of heating the preformed article having a large thickness, when the amount of radiant energy is increased on the surface of the material to be heated, the temperature difference increases between the surface and the interior. Usually, therefore, the heating is effected suppressing the amount of the radiant energy.
In an ordinary heating system based on a combination of the linear heaters, a relatively large distance is maintained between the heater and the material to be heated or the distance is finely adjusted among the heaters or between the heaters and the material to be heated, in order to uniformalize the level of radiant energy on the surface of the material to be heated. Therefore, laborious work is required for the adjustment to uniformly heat the material having relatively large surfaces to be heated, and the amount of radiant energy becomes relatively small on the surface of the material to be heated.
During the heating by the infrared rays, furthermore, the temperature of the material to be heated is also raised by the heat of convection in addition to radiant rays. The temperature rise by the convection is supported by the conduction of heat from the surface of the material to be heated, which works to increase the temperature difference in the direction of thickness of the material to be heated. In general, the temperature of the radiation member and the atmospheric temperature near the radiation body are very higher than the temperature of the material that is being heated. When the material to be heated is brought close to the linear heater, therefore, the effect of the heat of convection is added up to the distribution of radiant energy, and the temperature varies to a great extent in the material that is being heated. When the material to be heated is separated away from the linear heaters, on the other hand, the intensity of the radiant energy decreases on the surface of the material to be heated, whereby an extended period of time is required for the heating. When the heating is effected using an extended period of time, the temperature difference due to the heat of convection increases in the direction of thickness of the material to be heated.